Parents generally expect more self-control of teens than they really have, and teens will generally do what they can get away with, especially if they're compelled to going to Sunday School or to church.
At the start somewhere just announce it's time to put away your iPods, put your phones away, and we'll get to the lesson, or to the music, or something. Exceptions: if someone has their Bible on an iPod or iTouch, that's ok. But they need to remain focused on their Bible.
We often "bump" Bible readings, too. The leader gives a passage and "bumps" the passage reading to someone, "Bump Karen". Then Karen has to read at least one verse, and bump the rest to someone else. Hard to deal with that with an iPod in your ear.
"20 minutes is all they can stand", but they're quiet for about 50 minutes in school classes. If you integrate the impatience, find out what's making it happen, that can help. Is it lack of fellowship with others? Great, expand some time so they can get to know one another. Are they bored with things they've heard again & again? Hm, often that's because they aren't spurred to think about applying what they've heard. Are they simply skeptical it "works this way" because they have tried it? Maybe they're more involved -- thus have more mature questions -- about the way things work, or maybe they've "thought Christianity too hard and left it untried."
Lessons can be more interactive, more collaborative, more engaging too. They've gotta internalize the teaching, while school teaching is didactic and often just a firehose of information they can forget after a test. We let kids sit on window ledges, the floor, fidget, move around, but they also need to participate and push back. If you're not connecting, ask for relevant examples, what does Jesus mean by x when you're at school, what's Paul getting at with y when you're out at the mall, is this flying over your head.
